[TUT][HARD]Differential ADC For Monitoring Voltage Window

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Sometimes an ADC is needed to monitor only a voltage window. For example does the voltage of a 12V Pb-Battery never exceed 14.4V (V_MAX) and should never go below 10.5V (V_MIN). Voltages outside this range could damage the accumulator permanently. When using an ADC with single ended inputs you'll always monitor the complete voltage range of 0V to V_MAX. With differential ADC inputs it is possible to only monitor the desired voltage range of V_MIN to V_MAX. The advantage of this method is that you get a higher ADC resolution for the desired
input range. How this can be achieved is described in the attached pdf file.

Note that that the paper doesn't cover ADC functionality. Before reading you should have understood how an ADC with differential inputs operates.

In addition I have attached my "Electronics Calculator". This tool can help to select the right resistor values. How to use it is explained in the PDF file, too. The "Electronics Calculator" is a Microsoft Excel workbook. It contains macros. Thus you have to enable macros to make use of the tool. I have programmed the "Electronics Calculator" in MS Excel 2007. It may work with older versions. Maybe someone could check it and post his/her experience here.

I'm no native english speaker. So the PDF could be brimfull of nasty misspelling. It would be nice if someone could proofread the PDF and PM me the corrections. I'll update the PDF then.

Regards
Sebastian

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I have revised the complete paper. The new Version (V002) covers both ADCs with bipolar-differential inputs and ADCs with unipolar-differential inputs.

The effects of component tolerances are covered now.

Several real life examples were added. (ATtiny25 with internal/external reference).

I have extended the "Electronics Calculator" by a "Voltage Window" calculation feature. Now it is possible to design the voltage dividers without the need to understand the mathematical background.

Regards
Sebastian

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i'm getting error when tring to download the pdf file

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What kind of error? Were you logged in when you were trying to download it? (you have to be)

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Hello,

I do not understand one thing in pdf on page 18.

Why for calulating resolution for ADC in single ended mode you are using a formula 14,88V / 1024 . I think that resolution for adc only depends on ref voltage and the ADC bit resolution.

ADC_res = Vref / Number_of_ADC_bits

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There is a voltage divider on the input of the ADC to increase the measuring range from 2.56V to 14.4V.

To ensure a min measuring range of 14.4V you have to consider the resistor tolerance. That's why the requested example calculates with 14.88V.

Regards
Sebastian

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Hello again,

Thanks for quick answer a need to ask one more question in page 19 you use :

Vmin = 10,5 V
Vmax = 14,4 V

and Voltage Window Size is :

Vmax = 8,78 V;

In ideally situation voltage window size in my opinion will be :

Vmax - Vmin = 14,4 - 10,5 = 3,9 V , but because of tolerance of resistor, Vref you selected worst case scenario 8,78V.

Thanks again for support . :wink:

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Hi, can someone attach the PDF file again? I Am interested in this topic and this file is probably deleted. Thanks :)